1. Field of the Invention
The present invention relates to improved layered bismuth oxide ferroelectric materials and a process for fabrication of the materials into devices via the metal-organic decomposition method.
2. Description of Related Art
A new class of layered ferroelectric materials having a layered perovskite structure have begun finding use as thin ferroelectric films in radiation-hard, non-volatile microelectronic memories, high dielectric constant capacitors, energy storage devices, and the like. These layered materials comprise complex oxides of metals such as strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium, and thallium that spontaneously form layered crystalline lattices that include alternating layers of distinctly different sublattices, such as ferroelectric and non-ferroelectric sublattices. Generally, each layered material will include two or more of the above metals. For example, strontium, bismuth, and tantalum form the layered material strontium bismuth tantalate, SrBi.sub.2 Ta.sub.2 O.sub.9 (SBTO).
In addition to having good values of the ferroelectric parameters, it is also important that the physical quality of the ferroelectric films be suitable for use in manufacturing processes. For example, the film should have a relatively uniform grain size, which results in better thin film quality, i.e., films free of cracks and other defects. The film grain size should also be small compared to the thickness of the film; otherwise, the roughness of the film can be comparable to the thickness and other dimensions of the device components, which makes it difficult or impossible to fabricate devices within tolerances, with concomittant short circuits and other electrical breakdowns. Further, it is important that the fabrication processes be ones that can be performed relatively rapidly, since long processes are more expensive in terms of the use of facilities and personnel.
A variety of techniques has been disclosed for fabricating such layered ferroelectric materials; see, e.g., U.S. Pat. Nos. 5,434,102 and 5,439,845, both issued to H. Watanabe et al. The wafer is baked to dry the precursor and annealed to form a layered material on the wafer. Each metal in the ferroelectric material is formed separately as a carboxylate and the carboxylates are combined prior to application to the wafer. The carboxylates may be formed (in the case of strontium) by reacting the metal with a carboxylic acid. For the case of the other metals, these may be formed by reaction of a carboxylic acid with a metal alkyl or aryl or, more preferably, with a metal alkoxide.
The technique of Watanabe et al employs metallo-organic decomposition (MOD) to form the layered ferroelectric materials. These materials have the formula (rewritten) SrBi.sub.4-2x+.alpha. {(Ta.sub.1-y Nb.sub.y).sub.x (Ti.sub.z Zr.sub.1-z).sub.2-2x }.sub.2 O.sub.15-x 6x, where 0.ltoreq.x.ltoreq.1.0, 0.ltoreq.y.ltoreq.1.0, 0.ltoreq.z.ltoreq.1.0, and x-2.ltoreq..alpha..ltoreq.1.6(2-x).
In another technique, described in U.S. Pat. No. 5,316,579, issued to L. D. McMillan et al, fine mists of liquids are generated, using a rotating turbine blade disposed within an enclosure. A mixture of a liquid and a carrier gas are flowed into the enclosure such that it immediately impacts on the rotating turbine blade disposed near a lower end of the enclosure, and the resulting mist is withdrawn under vacuum near an upper end of the enclosure. The fine mists are used in the chemical vapor deposition of thin films of complex chemical compounds, such as ferroelectrics.
Applications of the layered ferroelectric materials include non-volatile memories (see, e.g., U.S. Pat. No. 5,406,510, issued to T. Mihara et al) and ferroelectric capacitors (see, e.g., C. A. Paz de Arujo et al, "Fatigue-free ferroelectric capacitors with platinum electrodes", Nature, Vol. 374, pp. 627-629 (13 Apr. 1995)).
Previous work in the art of ferroelectric materials has concentrated on the bismuth layered structure SrBi.sub.2 Ta.sub.2 O.sub.9 (SBTO). It has been shown that this material can produce devices with much improved retention of polarization and fatigue (useful number of cycles of polarization) as compared to other ferroelectric metal oxides (for example, lead zirconium titanate PZT!). Symetrix Corporation and its licensee, Kujundo Chemical of Japan, offer for sale a liquid source and process to construct BSTO material and devices. However, the inventors of the present invention have found that the supplied chemicals and process do not give devices with consistent electrical properties nor devices with sufficient properties at elevated temperatures.
In the patents and publications listed above, Symetrix Corporation describes liquid source materials and a process to produce bismuth-layered ceramic thin film ferroelectric devices. Both Symetrix, and their licensee (Kojundo Chemical) offer for sale the liquid source to the material, named "Y1", which is disclosed as the bismuth-layered structure SrBi.sub.2 Ta.sub.2 O.sub.9. According to the Symetrix disclosures, a liquid source containing a metal salt--organic acid mixture having the stoichiometry of Bi.sub.2.18 Sr.sub.0.99- Ta.sub.2.00 gives on processing the desired Y1 material. However, in the experience of the present inventors, the liquid source of Y1 supplied by either Symetrix or Kojundo does not on processing always give Y1 ferroelectric films of consistent electrical quality. Also, the Y1 thus formed, even in its best examples, does not have sufficient high temperature performance to meet the critical needs of the military and space systems hardware electronics applications.
Thus, a need remains to provide a liquid source that upon processing gives mixed metal oxide films of reproducible stoichiometry, and particularly ferroelectric films of consistent electrical quality and high temperature performance.